This invention relates to a silver and palladium powder for use in forming the buried electrodes in a monolithic ceramic capacitor by a standard ceramic/electrodes co-firing step.
The ceramic formulations used in early monolithic capacitors and even in some present day capacitors must be fired at a high temperature, typically 1400.degree. C., to achieve a full sintering and densification. Such capacitors include an all-noble-metal buried electrode such as pure palladium that does not lose conductivity through oxidation and that has a melting temperature (1550.degree. C.) greater than the above-noted high temperature at which the ceramic could be sintered to maturity. Lower melting metals would simply flow out of the ceramic during sintering. High melting base metals, such as iron could not be used because they would oxidize and become nonconductive in the oxygen firing atmosphere usually required to avoid reducing the ceramic. Furthermore, such metal oxides tend to diffuse into the adjacent ceramic degrading the electrical characteristics of that dielectric.
In the monolithic ceramic capacitor industry today, there is a pronounced move toward the use of lower-firing ceramic compositions that may include a sintering aid or flux, usually a glass. The glass first melts and sintering of the ceramic takes place by the simultaneous process of dissolution and recrystallization. This is known as liquid phase sintering, as is described in my patent U.S. Pat. No. 4,266,265, issued May 5, 1981, and assigned to the same assignee as is the present invention. Most such flux containing commercial capacitors are now sintered at below 1200.degree. C. and include a silver-palladium alloy electrode, typically 70Ag/30Pd by weight and having a solidous temperature of 1160.degree. C. Other ceramic compositions without fluxes sinter at such low temperatures, e.g. as described in my patent, U.S. Pat. No. 4,324,750 issued Apr. 13, 1982 and also assigned to the same assignee. These may also employ Ag/Pd alloy electrodes. A great savings is thereby realized through lower energy costs and lower metal costs. The temperatures at which the various silver-palladium alloys are always solid lie below the solidus curve 10 of FIG. 1.
A typical process for making such medium-firing monolithic ceramic capacitors includes mixing very fine silver and palladium metal powders (e.g. produced by a precipitation process) in an organic vehicle to form an electroding ink, forming a multiplicity of thin layers of green fluxed-ceramic, depositing a film of the silver-palladium ink on each of the green ceramic layers, stacking the inked ceramic layers and firing the stack to mature the ceramic and to alloy and sinter the electrode. Coarser silver powders are cheaper, are more readily available and are used in capacitors that have thick (e.g. 2 mils or 51 mm) ceramic layers between adjacent electrodes, in which case a lack of uniformity of the electrode surfaces is not so detrimental to capacitor quality. There is a trend in the industry toward thinner dielectric layers to increase the amount of electrical capacitance that can be got in a monolithic ceramic body of a given size. This requires smoother electrodes that will not randomly intrude into the thin dielectric layer and create points of low breakdown strength.
In any case, however, it often occurs at firing that a significant fraction of a production lot of capacitors is subject to cleaving that is usually a "delamination" at an interface between an electrode and the ceramic. This leads to a degradation in electrical properties of the capacitor, such as loss of capacity, and shorter life. Such cleaving and degradation becomes more severe as the thickness of the active dielectric layers becomes smaller.
It is an object of this invention to provide silver and palladium containing powders that may be used in electroding inks for making medium-firing monolithic ceramic capacitor bodies to provide more rugged capacitor bodies that do not delaminate or cleave in any fashion.
It is a further object of this invention to provide a method for making such capacitors that have thin dielectric layers and uniformly smooth buried electrodes.